netdev - RE: [PATCH 1/5] Add functions producing system time given a backing counter value

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Date:	Wed, 29 Jul 2015 16:05:19 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Hall, Christopher S" <christopher.s.hall@...el.com>
cc:	'John Stultz' <john.stultz@...aro.org>,
	Richard Cochran <richardcochran@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"Ronciak, John" <john.ronciak@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH 1/5] Add functions producing system time given a backing
 counter value

On Wed, 29 Jul 2015, Hall, Christopher S wrote:
> 
> > -----Original Message-----
> > From: John Stultz [mailto:john.stultz@...aro.org]
> > Sent: Monday, July 27, 2015 8:44 PM
> > To: Hall, Christopher S
> > Cc: Thomas Gleixner; Richard Cochran; Ingo Molnar; Kirsher, Jeffrey T;
> > Ronciak, John; H. Peter Anvin; x86@...nel.org; lkml;
> > netdev@...r.kernel.org
> > Subject: Re: [PATCH 1/5] Add functions producing system time given a
> > backing counter value
> > 
> > On Mon, Jul 27, 2015 at 5:46 PM, Christopher Hall
> > <christopher.s.hall@...el.com> wrote:
> > > * counter_to_rawmono64
> > > * counter_to_mono64
> > > * counter_to_realtime64
> > >
> > > Enables drivers to translate a captured system clock counter to system
> > > time. This is useful for network and audio devices that capture
> > timestamps
> > > in terms of both the system clock and device clock.
> > 
> > Huh.  So for counter_to_realtime64 & mono64, this seems to ignore the
> > fact that the multiplier is constantly adjusted and corrected. So that
> > calling the function twice with the same counter value may result in
> > different returned values.
> > 
> > I've not yet groked the whole patchset, but it seems like there needs
> > to be some mechanism that ensures the counter value is captured and
> > used in the same (or at least close) interval that the timekeeper data
> > is valid for.
> 
> The ART (and derived TSC) values are always in the past.  There's no
> chance that we could exceed the interval.  I don't think any similar
> usage would be a problem either.
> 
> Are you suggesting that, for completeness, this be enforced by the
> conversion function?
> 
> I do a check here to make sure that the current counter value isn't before
> the beginning of the current interval:
> 
> timekeeping_get_delta()
> ...
>                if (cycle_now < tkr->cycle_last &&
>                    tkr->cycle_last - cycle_now < ROLLOVER_THRESHOLD)
>                         return -EAGAIN;
> 
> If tkr->cycle_last - cycle_now is large, the assumption is that
> rollover occurred.  Otherwise, the caller should re-read the counter
> so that it falls within the current interval.  In my "normal use"
> testing, re-read never occurred.

Sure that never happens, because your rollover value is 2 << 39 for
whatever reasons.

So on a 1GHz machine that is (2 << 39) / 1e9 ~= 1099.51 seconds.

Oh well.

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